Automation systems are normally used for an automation, in particular an automated control and/or regulation, of industrial processes in technical plants, such as, for example steam power plants.
Automation systems of this type for technical plants, which are available in the form of software implemented in control systems, contain—in the form of single-loop or multi-loop control circuits—control systems by means of which the industrial processes can be mapped in these technical plants—usually at subsystem level—and by means of which the technical plant is run in a regulated/controlled manner by means of actuating elements/units controlled by the control systems.
Such a, for example single-loop, control circuit is made up of an analog measurement of a process variable (control variable) to be regulated and an input of the measured process variable into the automation system via an analog/digital (A/D) conversion. Furthermore, the control circuit usually provides a filter with which a noise of the measured process variable is eliminated. The control circuit specifies a desired value (target value) for the process variable and forms a deviation of the control variable from the desired value (control difference). From the control difference, a control algorithm (controller) determines how an actuating element/unit is to behave so that the control variable approximates the target value (actuating signal). The actuating signal is output via a digital/analog (D/A) conversion to the actuating element/unit of the technical system.
Moreover, when control circuits of this type are set up, control circuit parameters, such as transmission elements, time constants and/or amplification factors must be selected or set by the user.
For the automation of a subsystem of a technical plant, for example a steam power plant block in a steam power plant, it is necessary to set up a multiplicity of control circuits of this type for this subsystem that is to be automated.
The control circuits of the subsystem, for example a steam power plant block, are not independent from one another, but rather are strongly interlinked—due to industrial process circumstances (process engineering).
For example, the control of a pressure in a furnace of the steam power plant block via an induced draft is strongly influenced by the control of a fresh air feed via the forced-draft fan in the steam power plant block. Also, an increased fuel mass flow in the steam power plant block not only results in an increased steam production, but also influences the steam temperature in the steam power plant block which is to be kept constant by means of injections. The control of the feed water mass flow by means of the feed pump and the control of the feed water pressure by means of the feed water control valve are also dependent on one another.
In order to be able to achieve a high control quality and a high system stability, process-engineering-related (cross-) couplings of this type must again be decoupled or separated in the control engineering in the subsystem considered.
This is done via control engineering through the use of so-called decoupling networks with decoupling branches in the control structures or between the control circuits. These decoupling branches contain so-called decoupling elements, for example DT1 derivative lag elements and/or PTn delay elements.
Depending on the type of process engineering coupling actually present, decoupling must be carried out with a delay element or derivative lag element.
Via a derivative lag/delay branch with a derivative lag/delay element, a control difference of a specific process variable then no longer acts only on the actuating element allocated to it, but also on the actuating element of the coupled control circuit.
Due to the coordinated action with a plurality of actuating elements, it can be ensured that only the one process variable, instantaneously affected by a control difference, is influenced, and the other (process-engineering-coupled) process variables can remain at their desired value or deviate as little as possible and from the latter.
A design, i.e. a parameterization, of the decoupling branches is dependent on an actual dynamic process behavior of the systems considered, and must be carried out during a commissioning of the power plant control.
Plant tests are carried out during the parameterization. The evaluation of the test results then provides an insight into the parameters that are to be modified and to what extent. The parameters are then manually adjusted until the control achieves an optimum decoupling. The parameterization is laborious (time-consuming) and correspondingly expensive.
In the test performance period, the technical plant, for example the steam power plant, cannot be operated economically, for example at low cost according to a current power requirement of an electricity grid to be supplied with electric power by the steam power plant.
The dynamic response of the process is normally dependent on the current operating condition of the plant, so that the parameterization must be carried out in or for a plurality of operating points.
In addition, the dynamic behavior of the technical plant, and also the power plant process, will change due to the use of different fuel types, through wear, contamination and the like over time.
The decoupling branches, which have been set once to a specific plant behavior, then become no longer optimal with time. The control behavior will therefore deteriorate with time, and the stability of the plant will decrease.
On the basis thereof, the need therefore exists for a plant control which is simple to set up for coupled multivariable systems.
A so-called multivariable control for coupled multivariable system is known in the prior art.
In multivariable control, a complete system, such as a technical plant or only a subsystem of the technical plant, is considered with a plurality of control variables and a plurality of actuating elements. Here, every actuating element can—theoretically—act on every control variable, whereby a—theoretical—multi-dimensional process engineering coupling of processes can be taken into account in the subsystem.
A coupled multivariable system can thus be simulated in/by a multivariable control of this type.
If a multivariable controller is designed for a coupled multivariable system, decoupling structures are therefore also automatically generated.
However, a multivariable controller or multivariable control of this type has one or more of the following disadvantages which make it unsuitable for the control of technical plants, such as power plants.
The multivariable controller is based on a mathematical algorithm which cannot be represented in a function plan of a power plant control system. It is therefore not transparent and therefore not maintainable for a plant operator, i.e. is not modifiable and not extendable.
The result of the lacking transparency of the multivariable controller is furthermore that a commissioning engineer does not have the facility to set up additional structures with which special operating conditions can be taken into account.
However, non-linear boundary conditions of this type, such as, for example, limit curves of a pump, occur in every technical plant.
The multivariable controller can be designed once for a specific process structure and process dynamic response in the technical plant. However, it is not capable of adapting automatically to constantly changing boundary conditions.
The multivariable controller itself has parameters which are definable only with difficulty, in some cases only using a special tool. A targeted and desired reduction in the commissioning outlay cannot therefore be achieved.
The implementation of a multivariable controller additionally entails a high computing outlay and storage space requirement, and cannot therefore be used in an automation system, in particular in a power plant control system.
Since these disadvantages of multivariable controllers make their use unsuitable in the automation of technical plants or in power plant control systems, the use of single-loop control circuits with decoupling branches and the performance of plant tests for the parameterization of the decoupling elements is current practice.
Manual parameterization is currently restricted to the use of low-order decoupling elements, as there would otherwise be too many parameters to be set in the plant tests, which would ultimately be unmanageable in practice.
However, a lower control quality must in some cases be accepted as a result, since a higher-order decoupling element which is possibly more suitable in terms of control engineering could not be set manually.
It is furthermore known to implement automation systems, in particular plant engineering controls and/or regulations—mostly in the form of software—in a control system of a plant.
A control system, in particular a process control system, of a plant therefore normally designates means and methods which serve to control, regulate and secure such a process engineering plant.